Quality in Digital Cinema

In the first of this series of articles, I defined cinema as the art of presenting motion pictures. The term digital cinema simply means we are applying digital technology to the art. Cinematographers and studios demand a high quality level of their art when released to the cinema. However, the cinema is gradually becoming a place where more than motion pictures are shown. New forms of content termed alternative content, or specialized content, or electronic advertising are finding their way into the cinema. These new forms of cinema content are not always as demanding of quality as motion pictures. The difference in quality found in digital and electronic cinema systems is the topic of this article.

The quality of the cinema presentation system is generally dictated by the content provider. This is distinguished from consumer entertainment, where the quality of the presentation system is decided by the consumer. While it's true that the cinema owner generally has a range of equipment from which to choose, the quality level of this range is of a grade that meets the needs of the content provider. This is one of the ways in which cinemas differentiate themselves from the average home system. The high presentation quality is one of the cinema's strong marketing advantages.

In digital cinema, we see this principle exemplified by the strong effort made, by Digital Cinema Initiatives of Hollywood, towards establishing parameters for quality digital presentations of first release motion pictures. DCI, as it is known, was formed by seven major Hollywood studios in part to produce such recommendations. But high quality digital presentation systems are costly, and not all content providers agree that their needs require this grade of presentation. Some providers of electronically presented music concerts, sport events, plays, documentaries, and other specialized content are looking towards the use of more affordable, lower quality, systems for the presentation of their content. As an example, the Docuzone project in Europe, which will install 175 electronic cinema systems in 8 countries for the presentation of specialized content, is considering systems of lower quality than those found in trial digital cinema installations. In the US, low cost electronic cinema advertising systems, which by design provide a lower grade of presentation from digital cinema, will soon number 10,000. In some installations, these advertising systems are also used to present music concerts and sport events.

There are several factors that influence our perception of image quality. We often hear about image resolution, but quality is also affected by color, contrast, steadiness, and softness. Issues such as color and contrast are affected by good design, with good performance generally reflected in the higher cost of the technology. Electronic projection technologies do not suffer the image steadiness problem of film projectors, but the choice of electronic projection technology can have a strong effect on image softness, due to factors such as persistence, image resizing, and compression. Often overlooked, image quality is also affected by screen perforations. Cinema screens are perforated to allow sound to pass through the screen. But perforated screens are not ideal for either audio or image. They impose a rolloff in audio frequency response above 1KHz, and cause spatial interference with the pixel array of the electronically projected image, not to mention light loss. One method for dealing with this is to perforate the screen with a pseudo-random pattern, reducing the interface between perforation and pixel.

As one might expect, system quality generally centers on the choice of projector. In digital cinema, the projector choice has been limited, with only the DLP Cinema grade of projector meeting the needs of Hollywood content providers. In terms of resolution, DLP Cinema projectors are limited to 2K images, where there can be up to 2048 vertical lines. Cinematographers and studios are looking ahead to projection technologies capable of 4K resolution, or 4096 vertical lines. Projector companies such as JVC and Sony are working on such products using a technology called Liquid Crystal on Silicon (LCOS), as reported in a prior article in this series.

However, there's more to quality than image resolution. DCI has attempted to quantify some of the factors that affect perception through direct on-screen tests. In measurements made at the Digital Cinema Laboratory in Hollywood, DCI determined that people can distinguish 11 bits of color depth within a specific color space, resulting in their specification of 12-bits of color depth. (This means 12-bits per color, 36 bits of color for 3 primaries.) Color depth determines the number of gradations possible of a particular color primary. The color depth of consumer high-definition video, for example, is 8-bits. To add to the color capability of digital cinema, the XYZ color space has been specified for the delivery format. The XYZ color space includes all colors visible to the human eye, and is larger than the color space of film, which, in turn, is larger than the color space for high-definition video. This means that with digital cinema, it'll be possible to distribute images having more colors than what's possible with either film or video.

Sound is also an important element affecting our perception of quality in motion pictures. Draft SMPTE audio standards for digital cinema call for audio sampling rates of 48kHz and 96kHz. (SMPTE stands for the Society of Motion Picture and Television Engineers.) While 48kHz is the common sample rate in use today, the standard will require systems to provide compatibility with the higher sample rate, allowing content owners to release in this higher quality format. Audio will be delivered in an uncompressed mode, as opposed to the compressed digital audio now in use with film-based cinema. The number of audio channels available will also be increased to 16, although don't expect to see 16-track movies released any time soon. Screen perforations were mentioned earlier. Screens are perforated so that speakers can be placed directly behind the projected image, allowing the sound to appear to come from the picture itself. However, perforated screens are a problem for both image and audio. For audio, the screen appears as an acoustical filter above 1KHz. Eliminating the pixels will improve the digital image, but first a new means is needed to create the experience of sound coming from the picture without placing speakers behind the screen. Addressing the screen perforation issue is one of the challenges that lie ahead in the implementation of high image resolution digital cinema.

The needs of specialized content are perhaps best represented by work in the ITU-R TG-6/9 task group for Large Screen Digital Imagery, or LSDI as it's known. (ITU-R stands for the International Telecommunications Union, Radiocommunication sector, an international standards body.) The LSDI format provides for the use of both compressed 5.1 audio and uncompressed 2-track audio. The 2-track audio can support stereo matrixed audio, such as Dolby Pro-Logic™, which most cinemas are equipped to decode. While many productions will choose to use the compressed 5.1 audio format, the 2-channel format has proven itself popular for electronic advertising in the US. The image format of LSDI will follow existing standard formats for high-definition video, including color space and image resolution. The LSDI format is intended to support broadcast standards to open the cinema market to broadcast content providers.

As can be seen, the presentation of electronic images will bring a new experience to cinema entertainment. New sources of content will be available never before been seen in the cinema, while motion pictures will have the potential to display quality levels never before seen in the cinema.